1. Field of the Invention
The present invention relates to a curable composition for imprints. More precisely, the invention relates to a curable composition for micropatterning to give imprints, which is used in producing magnetic recording media such as semiconductor integrated circuits, flat screens, microelectromechanical systems (MEMS), sensor devices, optical discs, high-density memory discs, etc.; optical members such as gratings, relief holograms, etc.; optical films for production of nanodevices, optical devices, flat panel displays, etc.; polarizing elements, thin-film transistors in liquid-crystal displays, organic transistors, color filters, overcoat layers, pillar materials, rib materials for liquid-crystal alignment, microlens arrays, immunoassay chips, DNA separation chips, microreactors, nanobio devices, optical waveguides, optical filters, photonic liquid crystals, etc.
2. Description of the Related Art
Imprint technology is a development advanced from embossing technology well known in the art of optical disc production, which comprises pressing a mold original with an embossed pattern formed on its surface (this is generally referred to as “mold”, “stamper” or “template”) against a resin to thereby accurately transfer the micropattern onto the resin through mechanical deformation of the resin. In this, when a mold is once prepared, then microstructures such as nanostructures can be repeatedly molded, and therefore, this is economical, and in addition, harmful wastes and discharges from this nanotechnology are reduced. Accordingly these days, this is expected to be applicable to various technical fields.
Two methods of imprint technology have been proposed; one is a thermal imprint method using a thermoplastic resin as the material to be worked (for example, see S. Chou, et al., Appl. Phys. Lett. Vol. 67, 3114 (1995)), and the other is a photoimprint method using a photocurable composition (for example, see M. Colbun, et al., Proc. SPIE, Vol. 3676, 379 (1999)). In the thermal imprint method, a mold is pressed against a polymer resin heated up to a temperature not lower than the glass transition temperature thereof, then the resin is cooled and thereafter released from the mold to thereby transfer the microstructure of the mold onto the resin on a substrate. The method is applicable to various resin materials and glass materials and is expected to be applicable to various fields. For example, U.S. Pat. Nos. 5,772,905 and 5,956,216 disclose a imprint method of forming nanopatterns inexpensively.
On the other hand, in the photoimprint method where a composition for photoimprints is photocured by photoirradiation through a transparent mold or a transparent substrate, the transferring material does not require heating in pressing it against the mold, and therefore the method enables room-temperature imprinting. Recently, new developments having the advantages of the above two as combined, have been reported, including a nanocasting method and a reversal imprint method for forming three-dimensional structures.
For the imprint methods as above, proposed are applied technologies to nano-scale mentioned below.
In the first technology, the molded pattern itself has a function, and is applied to various elements in nanotechnology and to structural members. Its examples include various micro/nano optical elements and high-density recording media, as well as structural members in optical films, flat panel displays, etc. The second technology is for hybrid-molding of microstructures and nanostructures, or for construction of laminate structures through simple interlayer positioning, and this is applied to production of μ-TAS (micro-total analysis system) and biochips. In the third technology, the formed pattern is used as a mask and is applied to a method of processing a substrate through etching or the like. In these technologies, high-precision positioning is combined with high-density integration; and in place of conventional lithography technology, these technologies are being applied to production of high-density semiconductor integrated circuits and transistors in liquid-crystal displays, and also to magnetic processing for next-generation hard discs referred to as patterned media. Recently, the action on industrialization of the above-mentioned imprint technologies and their applied technologies has become active for practical use thereof.
As one example of imprint technology, hereinunder described is an application to production of high-density semiconductor integrated circuits. The recent development in micropatterning and integration scale enlargement in semiconductor integrated circuits is remarkable, and high-definition photolithography for pattern transfer for realizing the intended micropatterning is being much promoted and advanced in the art. However, for further requirement for more definite micropatterning to a higher level, it is now difficult to satisfy all the three of micropattern resolution, cost reduction and throughput increase. Regarding this, as a technology of micropatterning capable of attaining at a low cost, imprint lithography, particularly imprint lithography (photoimprint technology) is proposed. For example, U.S. Pat. Nos. 5,772,905 and 5,259,926 disclose a imprint technology of using a silicon wafer as a stamper for transferring a microstructure of at most 25 nm. This application requires micropatternability on a level of a few tens nm and high-level etching resistance of the micropattern functioning as a mask in substrate processing.
An application example of imprint technology to production of next-generation hard disc drives (HDD) is described. Based on head performance improvement and media performance improvement closely connected with each other, the course of HDD history is for capacity increase and size reduction. From the viewpoint of media performance improvement, HDD has realized increased large-scale capacity as a result of the increase in the surface-recording density thereon. However, in increasing the recording density, there occurs a problem of so-called magnetic field expansion from the side surface of the magnetic head. The magnetic field expansion could not be reduced more than a certain level even though the size of the head is reduced, therefore causing a phenomenon of so-called sidelight. The sidelight, if any, causes erroneous writing on the adjacent tracks and may erase the already recorded data. In addition, owing to the magnetic field expansion, there may occur another problem in that superfluous signals may be read from the adjacent track in reproduction. To solve these problems, there are proposed technologies of discrete track media and bit patterned media of filling the distance between the adjacent tracks with a non-magnetic material to thereby physically and magnetically separate the tracks. As a method of forming the magnetic or non-magnetic pattern in production of these media, application of imprint technology is proposed. The application also requires micropatternability on a level of a few tens nm and high-level etching resistance of the micropattern functioning as a mask in substrate processing.
Next described is an application example of imprint technology to flat displays such as liquid-crystal displays (LCD) and plasma display panels (PDP).
With the recent tendency toward large-sized LCD substrates and PDP substrates for high-definition microprocessing thereon, photoimprint lithography has become specifically noted these days as an inexpensive lithography technology capable of being substituted for conventional photolithography for use in production of thin-film transistors (TFT) and electrode plates. Accordingly, it has become necessary to develop a photocurable resist capable of being substituted for the etching photoresist for use in conventional photolithography.
Further, for the structural members for LCD and others, application of photoimprint technology to transparent protective film materials described in JP-A-2005-197699 and 2005-301289, or to spacers described in JP-A-2005-301289 is being under investigation. Differing from the above-mentioned etching resist, the resist for such structural members finally remains in displays, and therefore, it may be referred to as “permanent resist” or “permanent film”.
The spacer to define the cell gap in liquid-crystal displays is also a type of the permanent film; and in conventional photolithography, a photocurable composition comprising a resin, a photopolymerizable monomer and an initiator has been generally widely used for it (for example, see JP-A-2004-240241). In general, the spacer is formed as follows: After a color filter is formed on a color filter substrate, or after a protective film for the color filter is formed, a photocurable composition is applied thereto, and a pattern having a size of from 10 μm or 20 μm or so is formed through photolithography, and this is further thermally cured through past-baking to form the intended spacer.
Further, imprint lithography is useful also in formation of permanent films in optical members such as microelectromechanical systems (MEMS), sensor devices, gratings, relief holograms, etc.; optical films for production of nanodevices, optical devices, flat panel displays, etc.; polarizing elements, thin-film transistors in liquid-crystal displays, organic transistors, color filters, overcoat layers, pillar materials, rib materials for liquid-crystal alignment, microlens arrays, immunoassay chips, DNA separation chips, microreactors, nanobio devices, optical waveguides, optical filters, photonic liquid crystals, etc.
In application to such permanent films, the formed pattern remains in the final products, and is therefore required to have high-level properties of mainly film durability and strength, including heat resistance, light resistance, solvent resistance, scratch resistance, high-level mechanical resistance to external pressure, hardness, etc.
Almost all patterns heretofore formed in conventional photolithography can be formed in imprint technology, which is therefore specifically noted as a technology capable of forming micropatterns inexpensively.
It is an assumption that these applications form a good pattern, however, with regard to the photoimprint method in the patterning, it is necessary for a liquid curable composition to be sufficiently charged in a mold, and the composition is required to have a low viscosity. Furthermore, for industrial use, stability over time is important, and if the viscosity changes over time, the patternability changes, which thus leads to a problem that a good pattern cannot be mass produced. Further, if foreign bodies are generated over time, there occurs a problem that the foreign bodies themselves become pattern defects, or a problem that when a mold is forcibly imposed onto a curable composition while pressed, the foreign bodies destroy the mold, thereby continuously generating defects at a time of later imprint. As such, in the curable composition for imprints, stability over time is very important, but measures for improving the stability over time, and further, measures for making it compatible with product performances such as dry etching resistance and membrane strength have not been substantially discussed hitherto.
JP-A-2004-346125 and JP-A-2006-152115 describe an example in which a microlens array having the characteristics of a good refractive index is fabricated using a curable composition comprising a xylylene acrylate and a specific fluorene-type polymerizable monomer. However, JP-A-2004-346125 and JP-A-2006-152115 also did not examine stability over time.
As described above, in the industrial use of a imprints method, the patternability of the curable composition for imprints is important, and in particular, the ability to stably form a pattern even when a curable composition is left over time is important. Furthermore, in the curable composition for imprints, membrane characteristics according to the applications, such as etching resistance and pattern durability are required.
The present inventors have reviewed the stability over time for the curable compositions as described in JP-A-2004-346125 and JP-A-2006-152115 for the purpose of improving the stability over time, and as a result, they have found that the stability over time of the curable compositions was not sufficient. They have also found that the dry etching resistance of the curable compositions was not sufficient.